1. Field of the Invention
The present invention relates to a method of fabricating an optical integrated circuit and, more particularly, to a ridge type semiconductor laser of laterally-coupled distributed feedback (LC-DFB) and a method of manufacturing the same.
2. Description of the Related Art
Distributed feedback (DFB) semiconductor lasers are known as devices which can be used in the fields of optical communication systems such as optical CATVs, pumping light sources for SHG short-wave lasers for high-density information recording or small solid-state lasers, and optical measurement. The DFB semiconductor laser is used for a light source together with the other semiconductor devices.
Conventional distributed feedback semiconductor lasers are probably formed by using two or more steps of epitaxial growth. In a ridge type DFB semiconductor laser formed using two or more steps of epitaxial growth, a grating (diffraction grating) is provided in a laser waveguide layer and thereafter another layer is formed on the waveguide on an epitaxial growth basis.
Recently, in order to avoid the complicated epitaxial growth in two or more steps, the so-called single-growth distributed feedback semiconductor lasers have been developed which are fabricated using one single step of epitaxial growth, i.e., which does not involve any second epitaxial growth.
For example, in R. D. Martin et al. "CW Performance of an InGaAs--GaAs--AlGaAs Laterally-Coupled Distributed Feedback (LC-DFB) Ridge Laser Diode" IEEE Photonics Technology Letters, Vol. 7, No. 3, pp 244-246, March 1995, an InGaAs--GaAs--AlGaAs distributed feedback semiconductor laser is disclosed in which an active layer and a cladding layer are formed on a substrate by means of epitaxial growth; a ridge stripe is formed on the cladding layer; and a grating is provided on the top portion of the ridge stripe and on flat portions on both sides thereof. In methods of manufacturing such a laterally-coupled distributed feedback semiconductor laser, the grating is formed on the entire region of the substrate including the top portion of the ridge type waveguide by means of direct writing with electron beams (EB). In a DFB semiconductor laser, in general, a periodic structure is formed which undergoes a variation in shape having a period .LAMBDA. in tile direction in which the laser beam propagates. This results in a periodic variation of the index of refraction which in turn results in an increase in the reflectivity at a wavelength for which the phases of periodically reflected beams match (Bragg reflection), thereby causing laser oscillation. Therefore, the oscillation frequency of a distributed feedback semiconductor laser is determined by the period .LAMBDA. of the periodic structure and, in general, a single longitudinal mode is obtained if the equation .LAMBDA.=m.lambda./2n is satisfied where m represents an integral number; .lambda. represents the oscillation wavelength (in vacuum); and n represents the index of refraction of the laser medium. While oscillation generally occurs in the vicinity of wavelength of the Bragg reflection, the period A is determined considering the refractive indices, thicknesses, and aspect ratios of the In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y active layer and the cladding layer of the InP ridge stripe material, the reflectivity of the resonator (cleavage planes), and even the lateral optical coupling coefficient.
In the fabrication of such a DFB semiconductor laser, a longitudinally extending ridge stripe is formed by an etching process on the cladding layer and then the grating each line crossing the ridge stripe is formed on and around the ridge stripe. The dry etching process is almost employed for the method for forming the grating because it is excellent in controllability.
However, there are many failures in making the grating with a preferable shape on the side wall of the ridge stripe since it is difficult to make a mask proof against damage from the dry etching and to transfer the mask onto the side wall. The difficulty to fabricate the grating is a serious matter since the property of the DFB semiconductor laser is subject to the shape or status of each line in the grating at the foot of the ridge stripe. The dry etching process may also damage the surface of the substrate and thereby our apprehensions for inferiority of the property of the DFB semiconductor laser remains. On the other hand, the wet etching process hardly damages the surface of the substrate of semiconductor, but it is inferior in controllability because the property of the wet etching is apt to be subject to the crystal plane of semiconductor. The individual use of the wet etching is not suitable for the fabrication of the grating at the foot of the ridge stripe in the ridge type LC-DFB semiconductor laser.
Accordingly, it is very difficult to obtain a secure optical coupling between the grating and the guided light in the ridge stripe waveguide at the foot thereof in the conventional ridge type LC-DFB semiconductor laser.